Assessing the risk

The balance between risk and uncertainty is not easy to judge in a situation where known dangers can be transformed by biological change. There is no certain ground on which the government can pin its response to the outbreak of the H5N1 strain of avian flu on an industrial poultry farm last week. It must manage fears with one eye on the generally good record of past outbreaks and one eye on the much greater threat which may follow.

At worst, this threat is very great. The NHS contingency plan for an influenza pandemic warns that "around a quarter of the population could be affected with over 50,000 deaths in the UK alone". The infamous pandemic that followed the first world war (and which is said to have killed more people than all the battles that came before it) killed some 250,000 people in Britain. A repeat would disrupt life to an extraordinary extent. Even if the worst imaginings of scientists do not come to pass, the world may be on the edge of a grim decade.

But then again, it may not happen. Little is certain about how the disease will evolve. What is clear is that as things stand the risk to humans is low. Patricia Hewitt was right to point this out yesterday. So was Pat Troop, the head of the Health Protection Agency, who told the BBC that "the disease doesn't pass easily from bird to human". There have been tens of millions of cases of avian flu but fewer than 200 laboratory-confirmed deaths among humans.

Will this relatively secure situation last? If it does, avian flu poses little more risk than the outbreak of foot and mouth, which swept through British livestock in 2001. But as the World Health Organisation warns, "the virus has met all the prerequisites for the start of a pandemic save one: an ability to spread efficiently and sustainably among humans". Prone to rapid evolution, the current influenza virus could pass the test.

That is why it must be suppressed fast. The government aims to do this - although it was undermined by the slow response shown by the Bernard Matthews plant in Suffolk, which took 48 hours to report the disease. Yet that is nothing against the challenges faced by developing countries. If the disease is to mutate, it will most likely happen somewhere such as Nigeria (where the first recorded human death in sub-Saharan Africa occurred on Saturday).

Britain's industrialised poultry sector, once infected, aims to stamp out infection through rapid mass slaughter of flocks. Awful though that sounds, it should contain the present outbreak. But if, somewhere in the world, the virus mutates to transfer between humans, this might not be enough to prevent a global pandemic.